Slower than .NET `ValueStringBuilder` & two ideas how to fix it

[alex-jitbit]

I benchmarked ZString against .NET built-in ValueStringBuilder (not exposed publicly but we can steal extract the sources from https://source.dot.net/#System.Text.RegularExpressions/src/libraries/Common/src/System/Text/ValueStringBuilder.cs,42c1e0215cccb8b1 )

For some cases (when knowing length upfront and when appending 1-char strings) ValueStringBuilder is much faster.

Steps needed:

1. We should allow passing capacity to CreateStringBuilder to preallocate buffer if length is known
2. We should use an optimized path when Append is called with a 1-character string. Look at how ValueStringBuilder does it. If length is 1 it just sets buffer[pos++] = inputString[0] which is extremely fast, much faster than calling .CopyTo. https://source.dot.net/#System.Text.RegularExpressions/src/libraries/Common/src/System/Text/ValueStringBuilder.cs,42c1e0215cccb8b1